In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, Nov. 1999). Performance may be improved if both the transmitter and the receiver for a base station sector are equipped with multiple antennas (e.g., an sector antenna array) to provide a multiple-input multiple-output (MIMO) communication channel(s) for the base station sector. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from a sector antenna array over a MIMO channel to a wireless terminal in the sector, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from the base station sector antenna array for the sector. Stated in other words, multiple symbol streams may be transmitted from the base station sector antenna array for the sector to the wireless terminal over the same downlink transmission time interval (TTI) and/or time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station sector transmitting from the same sector antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station sector antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station sector antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
In a two layer MIMO transmission (2Tx) scheme, up to two layers/streams of information/data may be transmitted in parallel using a same TTI/TFRE. Four layer MIMO transmission (4Tx) schemes are proposed for High-Speed-Downlink-Packet-Access (HSDPA) within Third Generation Partnership Project (3GPP) standardization as disclosed, for example, in 3GPP RP-111393 and 3GPP R1-111763, the disclosures of both of which are hereby incorporated herein in their entireties by reference. Accordingly, up to 4 layers of information/data may be transmitted in parallel using a same TTI/TFRE when using 4-branch MIMO transmission.
Hybrid automatic repeat request (HARQ) may be used in wireless systems to overcome transmission errors that are not corrected using a forward error correcting code (also referred to as a channel code). In a typical implementation of a HARQ process, a cyclic redundancy check (CRC) code is attached to each data packet (also referred to as a transport data block or a data block) to be transmitted by a transmitter (e.g., a base station) for error detection. At the receiver (e.g., a wireless terminal), the contents of the each received packet (transport data block) may be validated using the attached CRC. If the received packet fails the CRC validation, the receiver sends a non-acknowledgement (NAK) signal back to the transmitter to request retransmission. A packet that fails CRC validation may be retransmitted until either the packet is decoded successfully or until a maximum number of retransmissions (e.g., 4 to 6 retransmissions) is reached. Otherwise, if the received packet is successfully validated (either after an initial transmission or a retransmission) using the CRC validation, an acknowledgement (ACK) signal is sent back to the transmitter to acknowledge correct decoding of data packet. At the receiver, a received retransmitted packet and the received initially transmitted packet (that failed CRC validation) may be combined to improve the system throughput. Depending on the way the packets are combined, HARQ systems may be classified into two categories, namely, Chase combining (CC) or Incremental Redundancy (IR).
Multiple antennas employed at the transmitter and receiver may significantly increase system capacity as discussed for example, by: (1) I. E. Telatar in “Capacity Of Multi-Antenna Gaussian Channels,” Eur. Trans. Telecommun., vol. 10, pp. 585-595, November 1999; and (2) David Gesbert, et al., in “From Theory To Practice: An Overview Of MIMO Space-Time Coded Wireless Systems,” IEEE Journal of Selected Areas in Commun., vol. 21, pp. 281302, April 2003. The disclosures of both of the above referenced publications are hereby incorporated herein in their entireties by reference.
By transmitting independent symbol streams in a same frequency bandwidth using spatial multiplexing (SM) as discussed above, a linear increase in data rate may be achieved with the increased number of antennas when operating at a relatively high signal to noise ratio. In a spatial multiplexing system, each transport data block (also referred to as a packet) may be mapped to a respective MIMO layer. For example, spatial multiplexing may be recommended in LTE/LTE-A with 2 antennas and for HSDPA with 2 antennas in DL and for UL, as discussed for example, by: (1) 3GPP, “Technical Specification Group Radio Access Network; Physical Layer Procedures (FDD) (Tech. Spec. 25.214 V7.7.0),” November 2007, available online at http://www.3gpp.org/ftp/Specs/html-info/25214.htm); and (2) 3rd Generation Partnership Project, “UTRA-UTRAN Long Term Evolution (LTE) And 3GPP System Architecture Evolution (SAE),” available at http://www.3gpp.org/Highlights/LTE/LTE.htm. The disclosures of both of the above referenced publications are hereby incorporated herein in their entireties by reference.
For a spatial multiplexing system with multiple codewords, there may be instances when only the wireless terminal reports ACK for a first transport data block and a NAK for second transport data block transmitted/received during a same TTI/TFRE. FIG. 6 shows the success probabilities in a 2×2 MIMO for a downlink channel for various wireless terminal speeds. As shown in FIG. 6, as the wireless terminal speed increases, the probability that only one transport data block passes and the other transport data block of the same TTI/TFRE fails increases. This increased probability of failure may be due to outdated CQI (channel quality information). When one transport data block passes and the other transport data block fails, the transmitter may retransmit the failed data block based on the ACK/NAK indications. Because the downlink channel conditions may not vary at a fast rate over two to three consecutive transmission intervals, however, a transmission quality of the transport data block that fails CRC validation in the previous transmission may not improve with retransmission. Accordingly, a relatively high number of retransmissions may be required to achieve successful decoding and CRC validation of a transmitted/retransmitted data block. Unfortunately, the relatively high number of retransmissions may introduce delay in transferring data blocks to higher layers. In addition, there may be an increased probability of a relatively high residual block error rate.